The frequency-controlled a.c. motor is the most advanced design, for instance for elevator motor drives. With frequency control the efficiency is high at all motor speeds, and the mains power factor is nearly unity. Frequency control is applicable in elevators with, as well as without, gear transmissions and at any speed. In addition, a simple and moderately priced short-circuit motor can be used. In elevator use, a transistor inverter implemented with transistors is best suited for frequency control since with transistors the highest switching frequency, among present power electronics components, is achieved. Also GTO thyristors are conceivable since their switching times are approximately of equal length, but because of the switch protection the main current circuit will be more complicated than with transistors.
Experience has revealed that a good strategy for controlling a short-circuit motor is to keep the magnetic flux constant. The magnetic flux is in general associated with slow response. If the magnetic flux changes, a time constant is introduced in the system. Stabilizing the magnetic flux can be achieved by keeping constant the magnetic flux of the stator, rotor or air gap. It is most advantageous to keep the magnetic flux of the stator constant since in that case the risk of the components of the magnetic circuit of the motor becoming saturated is least.
In present-day inverter technology, current feedback is employed for improving the curve shape, in which the phase currents obtained from the inverter output are measured. A negative feature of current feedback is slow response. This is due to the fact that in current feedback the inductances of the motor give rise to time constants. The control loop is usually the slower the greater the number of time constants. Moreover, current measuring elements are expensive because they are required to be able to measure direct current as well.
If pulse width modulation is effected in the inverter with the aid of a comparator as sinusoidal and triangular voltage comparison without feedback, the current supplied to the motor by the inverter is not sufficiently sinusoidal for instance in elevator use because in the rectified intermediate voltage circuit, from which the three-phase alternative voltage supplying the motor is obtained by rectifying with the inverter, the voltage is not constant, and because the semiconductor switch does not follow its driver without delay. A third factor causing error is the differential voltage which is caused by the residual voltage across the power electronics component, compared with the voltage appearing with the other direction of current, when the diode in parallel with the semiconductor switch is conductive. In practice, these errors cause vibration of the motor, which e.g. in elevator use impairs the performance of the elevator and causes passenger discomfort.